[meteorite-list] Ghubara Revisited

From: Dan Fronefield <dfronfld_at_meteoritecentral.com>
Date: Thu Apr 22 09:37:34 2004
Message-ID: <3.0.6.32.20001214200648.00b86b80_at_hiwaay.net>

Well, I couldn't find my loupe and my scanner is still dead so I can't send
pics. But visually (unaided) it still seems as if the the lighter matrix
has more defined chondrules. Obviously, this could be an artifact of
limited magnification.....near sightedness.....color blindness.....oh heck,
I give up.......when I poke it with my white cane, it all looks alike <G>.

I'll have to unpack my stereo microscope to take a "real" look at it.

So.....if Ghubara is prone to rusting, what is the prefered treatment? (I
have mine soaking in alcohol at the moment)

Thanks
Dan



At 12:00 AM 12/15/00 +0100, you wrote:
>Dan wrote:
>
>> The lighter colored section also seemed
>> to have more defined chondrules.
>
>Bernd responded:
>
>> a regolith breccia with a dark matrix and light
>> clasts. The lighter colored sections have more
>> defined chondrules because there are L3 frag-
>> ments set in an L5 host.
>
>James Baxter (private mail):
>
>> I recently obtained a slice of Ghubara in which the DARK
>> portion appears to be unequilibrated and has prominent
>> chondrules and the light portion looks more equilibrated
>> and has fewer and less distinct chondrules. This seems to
>> be the reverse of what you described.
>
>Hello List,
>
>James' observations are confirmed by a passage from Wasson's 1974 book
>on meteorites [WASSON J.T. (1974) Meteorites, Springer Verlag 1974, p.
>196]:
>
>"The difference in petrologic grade between xenoliths and host may be
>related to their relative friabilities. The material of lower petrologic
>type was more friable and was comminuted to form the host; the stronger
>materials of higher petrologic type were fragmented, but generally
>survived as larger clasts."
>
>Lower petrologic type obviously implies more chondrules - thus the dark
>Ghubara matrix should show abundant chondrules. James' scanned picture
>clearly shows just that and so does a look at my specimen. Higher
>petrologic type means a higher degree of recrystallization, and,
>consequently fewer chondrules. In a nutshell: The host matrix is
>chondrule-rich, the clasts should be chondrule-poor.
>
>But Dan wrote to the List:
>
>> The lighter colored section also seemed
>> to have more defined chondrules.
>
>Perhaps Wasson can again help (page 196):
>
>"If this explanation is correct, some cases should be found where the
>clast is of lower petrologic type than the host, since the correlation
>between friability and petrologic type is not a perfect one."
>
>Wasson does not mention if such "cases" may occur simultaneously in one
>and the same meteorite (specimen). Does James have the "run-of-the-mill"
>version of Ghubara and Dan the more "exotic" version? Or is Dan
>mistaken?
>
>With the help of a 12x loupe, I took another look at my Ghubara slice
>and noticed about an equal number of chondrules in both "lithologies",
>whereas James' xenolithic component almost seems to be devoid of any
>chondrules; so now I am at my wit's end :-(
>
>Folks, grab your Ghubaras, look at them closely, take your Ghubara thin
>sections and examine them in plane and polarized light and report to the
>list your findings. I think this is a great occasion for this list to do
>invaluable scientific, observational work.
>
>By the way, the entry in the Blue Book also supports James' observation:
>
>"... the host is unequilibrated, with olivine Fa21.6
>to Fa26.5, but olivine of xenoliths is Fa24 ..."
>
>
>Best wishes and
>thank you very
>much James,
>
>Bernd

Handmade Knives by D. Fronefield
"Specializing in meteorites and other exotic materials"
http://hiwaay.net/~dfronfld
Received on Thu 14 Dec 2000 08:06:48 PM PST